1. Field of the Invention
This invention pertains to integrated optical circuits having optical waveguides wherein a portion of the optical waveguides' refractive index is adjusted thermo-optically. More particularly, this invention pertains to thermo-optic control of the transfer function of a compound optical resonator by use of individual heaters localized to each resonator waveguide element and a local temperature sensor.
2. Description of the Related Art
The field of integrated optics has expanded tremendously over the past decade, and integrated optical device solutions are now being proposed for applications in a variety of fields including: telecommunications, data communications, high performance computing, biological and chemical sensing, and RF networks. Long haul, metropolitan, and local networks, as well as fiber-to-the-home applications, predominately rely on optical communications for data transmission. Wavelength division multiplexing (WDM) technologies have enabled a dramatic increase in the communications bandwidth of optical fiber. At channel spacings of 50 GHz, hundreds of signal channels are possible within the S-, C-, and L-bands of optical fiber covering the wavelength region from 1460 to 1625 nm. Some fiber-to-the-home applications utilize the O-band (1260-1360 nm) as well. More recently, optical data transmission has been investigated for computing systems to enable high bandwidth communications between processors, memory, and I/O. Improved RF system designs, such as phased array radars, with integrated optical devices in place of conventional microwave components have been shown to have higher potential bandwidths and target resolution. Lastly, spectroscopic applications of integrated optical devices in the ultra-violet and visible wavelength regions are being considered for biological and chemical sensors.
Integrated optics is the technology of integrating various optical devices and components to transport, focus, multiplex, demultiplex, split, combine, polarize, isolate, couple, switch, filter, modulate (phase or amplitude), detect, and generate light. Integrated optical devices combine several of these functions on a common chip or substrate. Examples include commercial planar lightwave circuits (PLCs), such as those that have been installed in WDM communication systems to multiplex and demultiplex optical channels on a fiber. More complex multiple layer PLCs have also been designed that incorporate waveguides and device circuitry on multiple planar layers interconnected using optical via technology. Newer integrated devices such as photonic integrated circuits (PICs) are now being developed by companies such as Infinera (Sunnyvale, Calif.) for high performance network operating systems and optical routing and switching applications.
Optical waveguides are the key building block of integrated optical devices. Optical waveguides are light conduits consisting of a slab, strip, or cylinder of dielectric core material surrounded by dielectric cladding materials of lower refractive index. FIG. 1 depicts an example planar optical waveguide 40, known in the art as a ridge waveguide, formed on a substrate or wafer 42 by the formation of a lower optical cladding 44; chemical vapor deposition, lithographic patterning, and etching of an optical core element 46; and lastly by surrounding the optical core element with an upper optical cladding layer 48. Other types of optical waveguide designs include rib, trench, filled trench, and strip-loaded waveguides. Typical lateral and vertical dimensions of the core elements in glass-based optical planar lightwave circuit waveguides lie between about 0.5 and 5 microns.
An optical waveguide or combination of optical waveguides can be assembled to form devices such as: optical resonators, arrayed waveguide gratings, couplers, splitters, polarization splitters/combiners, polarization rotators, mach-zehnder interferometers, multimode interference waveguides, gratings, mode transformers, delay lines, and optical vias. Devices such as these may then be combined or integrated on an optical chip to create an integrated optical device or planar lightwave circuit that performs one or more optical functions such as: multiplexing/demultiplexing, optical add/drop, variable attenuation, switching, splitting/combining, filtering, spectral analysis, variable optical delay, clock distribution, amplitude/phase modulation, polarization rotation, comb generation, and dispersion compensation.
Integrated optical devices and planar lightwave circuits can be fabricated on a variety of substrates or wafers. Some of the more common materials used are silicon, silicon wafers having silicon-oxide (SiO2) or thermal oxide layer coatings, and indium-phosphide (InP). Other materials considered for substrate or wafer applications include germanium, silica, fused quartz, sapphire, alumina, glass, gallium-arsenide, silicon-carbide, lithium-niobate, silicon-on-insulator, germanium-on-insulator, and silicon-germanium. Integrated optical devices or planar lightwave circuits may also be fabricated on or placed over preformed devices or circuits such as: one or more electrical devices (e.g. transistors), optical devices (e.g. mode transformers), microelectromechanical (MEMS) devices (e.g. mirrors), or optoelectronic devices (e.g. detectors, amplifiers, modulators, light emitting diodes, or lasers).
The material system most commonly used for planar optical waveguide devices is germanium doped silicon oxide SiO2:Ge. The waveguide consists of a SiO2:Ge optical core element, having a refractive index, that is surrounded by lower and upper cladding layers having smaller refractive indices. Typical cladding layer materials include air, polymer, silica (SiO2) and doped silicas such as phosphosilicate glass (PSG), borosilicate glass (BSG), borophosphosilicate glass (BPSG), and flourine doped silica (SiOF). The ability to shrink the dimensions of an optical circuit is generally limited by the refractive index contrast of the optical waveguides from which it is formed, where the refractive index contrast is defined as the ratio (ncore−nclad)/nclad. The smallest possible size of an optical device is constrained by the minimum allowable radius of curvature of its optical waveguides before incurring significant optical propagation loss of 0.5 dB/cm or more. Lower optical losses of less than 0.2 dB/cm are preferred. The higher the index contrast, the easier it is for waveguides to be routed on a chip using tight turns and small bend radii of curvature. For SiO2:Ge based optical waveguides, the maximum index contrast is limited to about 0.03 (3%), resulting in a minimum radius of curvature of at least 500 mm.
Silicon-oxynitride (SiON) is another doped silica that has been used for the fabrication of planar lightwave circuits (see, e.g., G. L. Bona, R. Germann, and B. J. Offrein, “SiON high refractive index waveguide and planar lightwave circuits,” IBM J. Res. & Dev. 47 (2/3), 239 (2003) incorporated herein by reference). SiON materials can be formed that are substantially transparent with an absorption loss of less than about 0.5 dB/cm in the wavelength region from 700 nm to 1650 nm. For the wavelength range from 1460 nm to 1625 nm, propagation losses of less than 0.2 dB/cm have been demonstrated. Low loss optical waveguides have been demonstrated having SiON core elements and SiO2 cladding layers with significantly higher refractive index contrasts than is possible with SiO2:Ge (see, e.g., U.S. Pat. No. 6,614,977). Optical waveguides with refractive index contrasts of about 0.17 (17%) can be fabricated from SiON core elements, having a refractive index of 1.7, and SiO2 cladding layers, having a refractive index of about 1.45 measured at a wavelength of 1550 nm. At this high index contrast, waveguides can be designed with radii of curvature as small as about 35 mm. For the SiON core elements, the highest potential refractive index value is 2, corresponding to silicon-nitride (Si3N4). Other materials being looked at for high index contrast waveguide applications include silicon, germanium, indium-phosphide, aluminum-oxide, silicon-oxycarbide (SiOC), and polymers.
A common optical element in most optical integrated circuits is a thermo-optic device, wherein a portion of the optical core waveguiding elements within the device is locally heated with respect to rest of the device by use of a local resistive heating element. Local heating of a waveguide shifts the phase of an optical signal within the waveguide by way of the thermo-optic effect on refractive index and is usually denoted by the change of refractive index with change in temperature or dn/dT. In addition, there can be a secondary contribution to the thermally induced phase shift from thermal expansion of waveguide dimensions. For example, an optical device such as a ring resonator has a set of resonant wavelengths given approximately by:li=2prn/i where r is the ring radius, n is the optical waveguide effective index, and i is an integer. The resonant wavelengths of the ring may be changed by locally raising or lowering the temperature of the ring waveguide, and the effective refractive index may be approximated byn=no+(dn/dT)DT where no is the effective index at the initial temperature To, dn/dT is the rate of change of refractive index with temperature at To, and DT is the net change in temperature T−To. For optical devices formed from SiO2:Ge, the dn/dT is typically just under 1×10−5° C. For optical devices formed from chemical vapor deposited SiON-based optical waveguides, the dn/dT is typically about 1.1 to 1.4×10−5/° C. depending on the composition. Applications of optical resonator thermo-optic devices requiring precise control of resonant frequency of better than ±1 GHz are not well suited to materials with high dn/dT magnitudes greater than about 5×10−5. A large dn/dT in this case is not desirable as it will require very precise local temperature control on the order of 0.1° C. Low dn/dT magnitudes of less than about 1×10−5, however, require somewhat more power as larger local temperature changes are needed to achieve a desired refractive index change, Dn.
Optical resonators are optical cavities supporting standing or traveling wave resonant optical modes. This invention considers an optical resonator to be any closed loop optical waveguide or disk supporting resonant optical modes. Examples of optical resonator waveguides 60 are depicted in FIG. 2 and include an optical waveguide core with a disk geometry 62, ring geometry 64, ellipse geometry 66, or racetrack geometry 68. Optical resonator waveguides 60 having the ring geometry 64 are the most commonly studied resonator cavity for integrated optical devices with a set of resonant wavelengths determined by the radius and effective index of the cavity. Optical resonators can be used singly or in multiples to form a compound optical resonator. Higher order cascades of optically coupled resonators are depicted in FIG. 3 showing second 80, third 82, and fifth 84 order compound optical ring resonator cavities. The Lorentzian response of a single-ring, channel dropping filter can be improved upon to achieve a wider and flatter passband as well as larger out of band signal rejection by moving to higher order filters, that is compound resonators consisting of multiple optically coupled resonators. The theory of compound optical resonators is discussed by B. E. Little et al. in “Microring resonator channel dropping filters,” Journal of Lightwave Technology, Vol. 15, No. 6, pp 998-1005 (1997) and is incorporated herein by reference.
Compound optical resonators must be optically coupled to at least one optical waveguide to form a useful optical device. A compound optical resonator cavity coupled to a single optical waveguide, as shown by 90 in FIG. 4, can be used to modify the phase of an optical signal in an all-pass filter or a dispersion compensator device. A compound optical resonator cavity coupled to two optical waveguides, depicted by 92 in FIG. 4, can be used as a channel dropping filter in which an optical signal or set of optical signals resonant with the modes of the compound optical resonator are dropped from one optical waveguide and added to the other. In these examples, the optical waveguide or waveguides must be adjacent to one of the resonator waveguides of the compound optical resonator such that the optical modes are coupled. In FIG. 4, optical waveguides are shown that are positioned laterally adjacent 92, above 94, and below 96 optical resonators.
The transfer function of an optical system is the effect on an optical signal in the frequency domain. The transfer function, for example, of an optical filter device acting on an optical input signal determines both the shape and center wavelength (frequency) of the optical output. A filter composed of a single ring resonator will take an optically flat input signal and transform it into a set of Lorentzian shaped output signals centered at the resonant wavelengths of the resonator. Optical filters composed of multiple and identical coupled ring resonators produce an output signal having a flatter and broader passband as well as larger out of band signal rejection.
The output of a thermo-optic compound optical resonator device will generally remain at a fixed wavelength unless there is a change in temperature. The temperature may be changed globally using a heater or Peltier element such as a thermoelectric cooler (TEC) that is in thermal contact with the integrated optical device substrate. It is more desirable, however, to use a heater or set of individual heaters localized to each compound thermo-optic element in the integrated optical device to allow for independent control. Simple resistive heaters can be fabricated by the deposition and patterning of metal films (such as platinum, gold, aluminum, chrome, nickel, nichrome, or tungsten) or semiconductor materials such as polysilicon. Other possible heater designs include localized Peltier elements.
Some commercially available thermo-optic devices such as switch arrays and optical attentuators operate by the simple application of a predetermined fixed power to each thermo-optic element heater in order to achieve a calibrated optical state. More sophisticated devices, such as those incorporating compound resonators, require a method of control that utilizes a feedback control loop. One possible feedback method is to analyze the optical output signal itself, by monitoring the signal's center wavelength for example, and use this parameter to determine the appropriate heater power dissipation applied to the thermo-optic element in real time. This method becomes unwieldy for a complex integrated optical device and would involve multiple real time optical measurements for the elements within the device. A much simpler approach is to use as a feedback variable for each heater a measurement of the local temperature. The most simple device used for temperature measurement are thermistors or resistive temperature devices (RTDs) that have a resistance value that changes as a function of the local temperature. For example, a metal film resistor can be deposited around or over an optical resonator waveguide in order to monitor the local average temperature. Platinum is the most common metal used to fabricate metal film RTDs, but copper, nickel, and nickel-iron are also used. Thermistors are generally made from semiconducting metal-oxide ceramics. Other example temperature sensors include a temperature sensing diode or a thermocouple, which generates an electric potential between two dissimilar metals that is a function of temperature
A control system is required to adjust, regulate, or control the power dissipation in a heater. Generally, a control system will monitor the feedback variable from a temperature sensor (e.g. the resistance value of an RTD) and change the power dissipated in the heater as needed to bring the feedback variable to a predetermined set value or setpoint. Example controllers include simple on/off, proportional, proportional bandwidth, and PID type controllers. Examples of these controllers are commercially available from vendors such as Omega Engineering Inc., Stamford Conn. Integrated optical devices, however, generally integrate the control functions and electronic devices within a printed circuit board. The controllers can adjust the heater power dissipation through an analog change in voltage (current) applied to a heater or through a change in the duty cycle of a digital pulse train, or filtered digital pulse train, known as pulse width modulation (PWM). PWM controllers are commercially available as digital signal processor chips from companies such as Freescale Semiconductor Inc., Austin Tex. (e.g. DSP56F8XX series). Often these functions are integrated into a printed circuit board as well.
Prior art work has focused on methods of thermally tuning optical resonators composed of a single ring. In P. Heimala et al., “Thermally tunable integrated optical ring resonator with poly-si thermistor,” Journal of Lightwave Technology, Vol. 14, No. 10, pp 2260-2267 (1996), incorporated herein by reference, the authors disclose a Si3N4 rib-waveguide-based ring resonator integrated with a local thermistor and polysilicon heater. In this paper, the resonant wavelength is adjusted using the feedback of a thermistor to regulate the power dissipated in a heater overlaying a ring waveguide. U.S. Pat. No. 6,636,668, incorporated herein by reference, also discloses a thermally tunable resonator device, each resonator comprising a heater and a temperature sensor. Compound resonators comprising multiple rings are discussed in U.S. Pat. No. 6,411,752, incorporated herein by reference, where the use of localized heaters placed above or below each ring is proposed as a method of tuning, however, there is no mention of feedback control methods.
Although it is relatively straightforward to control the resonant wavelength of a single ring resonator using a heater and a temperature sensor, compound optical resonators comprising at least two optically coupled resonators are more complex. One could simply devise a design incorporating one large area heater that covered all the optical resonators of the compound resonator. The resonant wavelength of the compound resonator could then be adjusted using feedback from a single temperature sensor to control the power sent to the one heater. There are two major limitations of this design. One, a large area heater will use significantly more power than an individual heater that is substantially of similar size and conformal in shape to the optical resonator waveguides. Two, it is difficult to devise a large area heater that produces a laterally uniform temperature profile across a set of coupled resonators and over a reasonably large temperature range of many tens of degrees Celsius. This type of heater will generally be hotter in the center and cooler near the edges resulting in an offset in resonant wavelength, or detuning, between optical resonators positioned nearer the center and nearer the edge of the heater. The result would be a deterioration in the compound filter shape from optimum.
An alternate method would be simply to use independent control for each resonator in the compound resonator as disclosed by Kuipers et. al. in “Integrated optical signal handling device,” International Publication No. WO 02/103448 A2. There are several major disadvantages with this method. First, each optical resonator within the integrated optical device would need its own heater and temperature sensor. This greatly increases the number of electrical connections required by the optical chip. In addition, a complete feedback loop is required for each individual heater with electronics and software dedicated to the monitoring of each temperature sensor over time. An integrated optical chip having four compound resonators each consisting of five rings would require twenty temperature sensors and feedback loops. Second, independent feedback control loops localized to each ring will be influenced by the control loop operating on adjacent optical resonators. The optical coupling requirement between resonators causes a portion of the outer surfaces of each resonator waveguide to be positioned within about a micron or less of each other. This separation is known as the coupling gap. The small separation means that there will be significant thermal crosstalk between the temperature sensor of one resonator and the heaters of adjacent resonators that results in control loop instability.